The Resolution Dependence of Explicitly Modeled Convective Systems

The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified. Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slower, with the resultant mature mesoscale circulation becoming stronger than those produced in the 1-km case. It is found that the slower evolution in the coarse-resolution simulations is largely a result of the delayed strengthening of the convective cold pool, which is crucial to the evolution of a mature, upshear-tilted convective system. The relative success in producing realistic circulation patterns at later times for these cases occurs because the cold pool does eventually force the system to grow upscale, allowing it to be better resolved. The stronger circulation results from an overprediction of the vertical mass transport produced by the convection at the leading edge of the system, due to the inability of the coarse-resolution simulations to properly represent nonhydrostatic effects.